There is an increased interest in fiber-based access technologies. One reason for that is a growing demand for higher speeds in the access network (100 Mbps per user) to enable triple play (services such as voice, video, data, High Definition Television (HDTV) and interactive gaming). Other reasons are the higher competition that network operators are faced today on the broadband market and the price erosion on optical components. Several operators are considering deploying, or actually started to deploy different forms of fiber access network systems.
Fiber To The x (FTTX) is a generic term for any network architecture that uses optical fiber to replace all or part of the usual wire (normally copper wire) local loop used for telecommunications. As shown in FIG. 1, depending on how far the fiber is pulled toward the user, different scenarios are distinguished. These are fiber to the node or the cabinet (FTTN/FTTCab), fiber to the curb or the building (FTTB/FTTC) and fiber to the home (FTTH). These are the abbreviations in FIG. 1:                HN—Home Network        AN—Access Network        UNI—User Network Interface        SNI—Service Node Interface        ONU—Optical Network Unit        ONT—Optical Network Termination        OLT—Optical Line Termination        NT—Network Termination        CP—Customer Premises        CO—Central OfficeFTTCab        
A first step in replacing copper with fiber is to place a cabinet in the vicinity of the user to shorten the copper distance and thereby allowing higher-rate DSL transmission modes like VDSL2 (Very high speed Digital Subscriber Line). Today rate-limited ADSL2+ (Asymmetric DSL) technology is widely deployed to span long distances between the central office (point of present) and the user premises.
FTTB/C
In case the fiber is pulled towards the building/business or curb (multi-dwelling units, business parks), a very short copper-length (house wiring) has to be bridged by DSL (fiber to the building/business FTTB and fiber to the curb FTTB), see 1. The rational behind this kind of architecture is that fiber deployment becomes more expensive, the closer it reaches towards the user.
FTTH
In case copper is totally replaced by fiber from the central office to the user (fiber to the home FTTH) the migration to all-optical access finished, see FIG. 1.
FTTB/C/Cab architectures are copper-centric since they still use parts of the copper line to bridge the very-last mile, whereas FTTH is called fiber-centric since no copper is involved. One main technologies used for these scenarios is VDSL2, used in FTTCab, FTTC and in some FTTB deployments. Another is Gigabit Passive Optical Network (PON) or Ethernet PON backhaul, used in FTTcab, FTTB, FTTC and FTTH. PON will be described later.
VDSL2 (Very High Speed Digital Subscriber Line 2) is an access technology that exploits the existing infrastructure of copper. It can be deployed from central offices, from fiber-fed cabinets located near the customer premises, or within buildings. VDSL2 is the newest and most advanced standard of digital subscriber line (DSL) broadband wireline communications and is designed to support the wide deployment of Triple Play services. VDSL2 enables operators and carriers to gradually, flexibly, and cost-efficiently upgrade existing xDSL-infrastructure.
For FTTB/C/Cab and FTTH the optical network can consist either of a point to point network (active Ethernet) or a point to multipoint network (PON).
For FTTB/FTTC/FTTCab active equipment needs to be installed between the central office and the user premises in order to convert between fiber-transmitted optical signals and wire-transmitted electrical signals. Such active equipments will be described later. For FTTB, the equipment can be powered directly via house power lines whereas for FTTC and FTTCab power need to be provided by new installation.
The bit rate performance and coverage over time of copper and fiber centric FTTx evolution path differs. In a copper-centric solution positive outcomes are that wire is reused, the time to market is reduced and it is progressive with DSL innovation. Negative outcomes are many small nodes and risk against fast bandwidth evolution. In a fiber-centric solution positive outcomes are scalability in capital and operational expenditures, CO consolidation and bandwidth evolution. Negative outcomes are upfront civil works, longer time to market and cabling in buildings.
In a rehab scenario the copper-centric path includes an intermediate step via VDSL2 to provide services in the area of 50 Mbit/s with short time-to-market and reduces capital expenditures compared to the fiber-centric infrastructure. Clearly for greenfield, FTTH is the most economical choice. Most operators investing in FTTx are interested in an FTTCab solution due to cost reasons.
Optical networks can be split into two families depending on the whether the Optical Distribution Network (ODN), i.e. the fiber network between the Optical Line Termination (OLT) at the central office and the Optical Network Termination (ONT) in case of FTTH or Optical Network Unit (ONU) in case of FTTB/C/Cab close to the user, contains active equipment or not. See FIG. 1. In case the ODN is totally passive, the system is called passive optical network (PON) and mainly exist in a point to multipoint (p2 mp) architecture. Points to point (p2p) structures are also available (fiber-based Ethernet) which are active.
In order to operate PON, optical splitters (see FIG. 2) are used to enable a single optical fiber to serve multiple premises, typically splitted into 32 or 64 fibers depending on the manufacturer. Such a splitter cannot provide any switching or buffering capabilities. For PON, the ONU/ONT must perform some kind of special functions, such as (due to the absence of switching capability) broadcasting each signal leaving the OLT to all users served by that splitter. Moreover, since the splitter cannot perform buffering, each individual ONT must be coordinated in a multiplexing scheme to prevent signals leaving the customer from colliding at the intersection.
In comparison with active optical networks, PON has some advantages. They avoid the complexities involved in keeping electronic equipment operating outdoors, for instance caused by the need for powering. They also allow for analog broadcast which can simplify the delivery of analog television. A disadvantage is that the central office must contain the OLT, which is a particular powerful piece of transmitting equipment.
PONS have gained great attention in the last years due to the low cost (p2 mp implies a fiber-frugal tree-topology), maintenance (no remote powering in the FTTH configuration) and failsafe performance advantages (high meantime between failure, no active parts).
The fundamental architecture of a PON system is shown in FIG. 2. Some of the abbreviations have been explained in connection to FIG. 1. Other abbreviations are:                NE—Network Element providing additional services over the same ODN (fe cable-TV video overlay option) on a different wavelength in addition to GPON, OLT and the ONU.        AF—Adaptation Function        S—Point on the optical fiber just after the OLT/ONU (downstream/upstream) optical connection point.        R—Point on the optical fiber just before the OLT/ONU (downstream/upstream) optical connection point.        Tref/Vref—Reference points.        A/B—Points used if WDM-overlay is used.        OS—Optical Splitter        ANSMF—Access Network System Management Functions.        SNF—Service Node Function        
The OLT broadcasts data downstream to all ONU/Ts via the ODN using Time Division Multiplexing (TDM). In the upstream, different ONU/Ts are using granted timeslots to communicate data to the OLT via a Time Division Multiple Access (TDMA) scheme controlled by the OLT. Upstream and downstream are separated on different wavelengths. Video-overlay on a separate wavelength (Wavelength Division Multiplying WDM) is a supported feature by most PONS. The ODN consist of a common trunk fiber, the passive power splitter (OS) forking up to different users, and user-individual drop-fibers. The splitter is mostly placed in the field at a remote node (RN).
There are currently three alternative PON implementation schemes, the three major ones being Ethernet PON (EPON), broadband PON (BPON), and gigabit PON (GPON), see FIG. 3. All of these follow the topology shown in FIG. 2 but differ clearly in their transmission protocols and performance. As can be seen, GPON as a successor of BPON is the most advanced system in terms of protocol support, the rates (2488 Mbps down and 1244 Mbps up in practical systems), the total span (trunk plus drop span) and the number of users per OLT (split-ratio).
In case a FTTC or FTTCab solution is chosen, new small street-side cabinets need to be installed including the ONU. These cabinets typically hold the following components:                ONU (GPON) or Ethernet Switch (Active Ethernet): Terminating the fiber feed towards the central office.        IPDSLAM (VDSL2 or ADSL2+) and main distribution frame (MDF): Copper connection panel and access multiplexer.        Power unit: Power for the fiber/copper transmission equipment.        
Powering an FTTCab solution is one of the main cost drivers for this kind of deployment options. Since a high number of cabinets need to be newly installed, power is mostly not available and need to be encompassed. A typical cabinet supports from 12 to 96 users whereas a classical central offices is serving from 1000 to 10000 users, a difference of two orders of magnitude. The costs for cabinets including equipment and installation have to be traded-off against the gain that is achieved by re-using the copper in the very-last mile. Cabinet equipment also needs to be hardened (higher temperature range, no cooling). Thus low-cost cabinets are vital for FTTCab to be a successful option.
A lot of resources have been invested to design VDSL2 equipment, which main potential field of application is the combination with optical backhaul in an FTTCab solution. To compete with fiber-centric FTTx options, the solution has to be cost efficient.